So a player starts out with one character, the overlord and the game is like a huge real time strategy.

The one character has to protect the entire populace as they begin the building stage as the first wave of barbarian attacks attack.

After clearing it and making a name for the hero/heroine he will be awarded with some guard which he can strategically place and some towers.

After given some time the character can grow the simple village into a town and since its like a medieval warring states era he becomes a captain of the guard or captain of a raid.

The game goes into a samurai warriors style gameplay of one hero fighting hordes of enemies and his army can be upgraded after each chapter as the town continues to grow.

Of course the battle between states continues as your area grows, you can meet allies and increase the kingdom and be awarded a place and title and select another hero or play the main story as it grows into epic proportions from having a hidden bloodline to an ancient battle between good and evil.

He will play a huge role as the game progresses whether to make trade or just conquer another place.

And to make things more epic, he can use a weapon that changes form and can get exp from which form it is used most and unlock more additional combos or charge attacks.

I'm thinking this will just be a prologue and have it more epic and after he dies he gets another role to play as another story goes on as he becomes a worthy candidate and a Valkeiry gets him for another epic war in asgard.

sounds interesting, but also something we've seen in many games of the past. You have to think, probably, something unique to include in the story. I mean, you can keep the story, but why people should buy your game? it must be unique someway, so what features make it unique?

Sounds like it's got a lot of potential. Even the breif mechanics you mentioned have a lot of potential, but I wouldn't focus on depth of each system so much as interplay between them and making sure each works solidly. It would be easy to add a bunch of units and towers, but it would be better to work with a few interesting combinations and add more later since each system will take some serious effort to complete. Remember to move on quickly and don't get bogged down on any one area of the project.

Check out some links on TvTropes - the site can help you write better once you do enough research on it of course. It's all very straight forward, but there's a lot of content so it can be a bit of a challenge. If you have a plot idea, it's good to look it up and understand the successes and failures that have come before and used the same idea.

In you're case, you might want to take a special look at this page on TvTropes, because your plot concept might be an inversion of this if you play the feudal overlord. Consider playing on that fact to make the plot more interesting.